Field-tunable toroidal moment in the chiral-lattice magnet BaCoSiO₄

A toroidal dipole moment appears independent of the electric and magnetic dipole moment in the multipole expansion of electrodynamics. It arises naturally from vortex-like arrangements of spins. Observing and controlling spontaneous long-range orders of toroidal moments are highly promising for spintronics but remain challenging. In this talk, I will introduce our resent work on a chiral triangular-lattice magnet BaCoSiO₄ where a vortex-like spin configuration with a staggered arrangement of toroidal moments, a ferritoroidal state, is realized. Upon applying a magnetic field, we observe multi-stair toroidal transitions correlating directly with metamagnetic transitions. We establish a first-principles microscopic Hamiltonian that explains both the formation of toroidal states and the metamagnetic toroidal transition as a combined effect of the magnetic frustration and the Dzyaloshinskii-Moriya interactions allowed by the crystallographic chirality. I will also briefly touch on the remarkable dynamical response of this magnet where a strong resonance-like mode coexists with coninuous excitations.